Idiopathic thrombocytopenic purpura (ITP) results from an immunologic reaction against an autologous platelet-associated antigen(s) with subsequent antiplatelet antibody production with resultant platelet sensitization and phagocytosis. The present proposal has two goals: (1) To apply the techniques, developed thus far in our laboratory, for evaluating the site of origin and magnitude of production of antiplatelet antibody to the study of a large series of ITP patients. (2) Develop new methods to better evaluate the platelet antigen(s) involved and the secondary events which follow the binding of antibody and platelets. IgG production by cells from the spleen, bone marrow, and lymph nodes from ITP patients will be quantitated using the Fab-anti Fab assay system and the percentge of the total IgG which is platelet specific will be determined. The total quantity of platelet-specific IgG will be calculated by correcting for the number of cells per organ and the quantity required for saturation of platelet antigenic sites determined. In addition, the IgG associated with ITP platelets in vivo and the serum levels of platelet-binding IgG will be measured. These data will be correlated with the patient's clinical presentation and response to therapy. Platelet antigens will be studied by reacting radiolabeled platelet surface proteins with known antplatelet antibody and evaluating the fractions precipitated with goat antihuman IgG. This will provide an approach to characterizing and purifying the antigens involved in ITP. In addition the degree of chemotaxis, complement fixation, and platelet phagocytosis resulting from antibody-platelet interaction will be investigated.